[PATCH] perf/Power: PERF_EVENT_IOC_ENABLE does not reenable event
paulus at samba.org
Thu Jan 24 16:05:05 EST 2013
On Fri, Jan 11, 2013 at 11:11:17AM -0800, Sukadev Bhattiprolu wrote:
> If we disable a perf event because we exceeded the specified ->event_limit,
> power_pmu_stop() sets the PERF_HES_STOPPED flag on the event.
> If the application then re-enables the event using PERF_EVENT_IOC_ENABLE
> ioctl, we don't seem to ever clear this STOPPED flag. Consequently, the
> user space is never notified of the event.
> Following message has more background and test case.
> The problem reported there does not seem to occur on x86. My unverified theory:
> Both x86 and Power clear the event->hw.state flag to 0 in their ->pmu_start()
> operations. On X86 x86_pmu_start() is called from x86_pmu_enable(). But on
> Power, power_pmu_start() is not called from power_pmu_enable().
This code has changed a lot since I worked on it, but it seems like
x86 has the STOPPED flag set whenever the event isn't currently active
on a hardware counter, whereas we have it set only when the event has
> Used the following test cases to verify that this patch works on latest PAPI.
> $ papi.git/src/ctests/nonthread PAPI_TOT_CYC at 5000000
> $ papi.git/src/ctests/overflow_single_event
> Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> arch/powerpc/perf/core-book3s.c | 8 ++++++++
> 1 files changed, 8 insertions(+), 0 deletions(-)
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index aa2465e..a6faada 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -880,8 +880,16 @@ static int power_pmu_add(struct perf_event *event, int ef_flags)
> cpuhw->events[n0] = event->hw.config;
> cpuhw->flags[n0] = event->hw.event_base;
> + /*
> + * If this event was disabled in record_and_restart() because we
> + * exceeded the ->event_limit, this is probably a good time to
> + * re-enable the event ? If we don't reenable the event, we will
> + * never notify the user again about this event.
> + */
The comment seems a bit tentative. :) If the PERF_EF_START bit is set
then we are being told to restart the event.
> if (!(ef_flags & PERF_EF_START))
> event->hw.state = PERF_HES_STOPPED | PERF_HES_UPTODATE;
> + else
> + event->hw.state &= ~PERF_HES_STOPPED;
This looks fine, though I think you could equally well just set
event->hw.state to 0 in the else clause. That would clear the
UPTODATE flag too, which is appropriate since we are about to put the
event on a hardware counter.
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